1. Field of the Invention
The present invention relates to a power supply circuit and a control method of the same and, particularly, to a power supply circuit using a battery as a power supply and a control method of the same.
2. Description of Related Art
In mobile apparatus, a variety of devices such as a display device are driven using a power supply voltage that is supplied from an internal battery. Battery life is one of the most important considerations when a user chooses a mobile apparatus. Thus, a mobile apparatus with long battery life is advantageous in terms of apparatus choice. Recently, various developments have been made to elongate the battery life of mobile apparatus.
Although only one power supply voltage is supplied from a battery, a mobile apparatus incorporating a display device generally requires a plurality of voltages including a voltage higher than the power supply voltage of the battery in order to drive the display device. The voltage higher than the power supply voltage from the battery is generally generated using a boosting circuit. The boosting circuit receives the power supply voltage from the battery and generates an output voltage that is higher than the power supply voltage.
The power supply voltage of the battery decreases while driving the mobile apparatus with the battery. Accordingly, the output voltage of the boosting circuit to which a multiplication factor is set on the basis of the initial power supply voltage of the battery decreases as the voltage of the battery decreases. When the output voltage of the boosting circuit falls below a prescribed voltage value that is necessary for driving the mobile apparatus, the mobile apparatus stops operating, considering that the battery life runs out.
Even though the mobile apparatus stops operating, energy remains in the actual battery because the output voltage of the boosting circuit merely becomes lower than the prescribed voltage value in this state. Therefore, the battery life can be elongated by increasing the boosting factor of the boosting circuit. However, if the voltage is boosted with a large multiplication factor from the beginning, it is necessary to increase the dielectric strength of parts to which the boosted voltage is supplied, which is not economical. As an approach to such an issue, a technique for elongating battery life by increasing the multiplication factor of the boosting circuit when the power supply voltage of the battery becomes lower than the prescribed value is disclosed in Japanese Unexamined Patent Publication No. 2002-32131, for example.
The power supply circuit according to prior art which is disclosed in Japanese Unexamined Patent Publication No. 2002-32131 is described hereinafter with reference to FIG. 6. FIG. 6 is a block diagram showing the configuration of the power supply circuit according to prior art. FIG. 6 shows the configuration of a liquid crystal display device, which is a component of an electronic clock, and the configuration to supply a power supply voltage to the liquid crystal display device. As shown in FIG. 6, a liquid crystal display device 204 includes a liquid crystal display panel 21, a liquid crystal driving circuit 22, a power supply control circuit 23 that supplies a driving voltage Vz for driving the liquid crystal display panel, and a drive control circuit 24 that controls the liquid crystal driving circuit 22 based on display data and various kinds of control data which are supplied from a control device 11 such as a CPU that controls an electronic clock 200 as a whole. FIG. 6 shows the configuration where a secondary battery 10 supplies a power supply voltage not only to the liquid crystal display device 204 but also to other loads such as an illumination unit 301 and a vibrating motor 302.
The liquid crystal driving circuit 22 includes a scan driving circuit 22a that drives a scan electrode (common electrode) of the liquid crystal display panel 21 and a signal driving circuit 22b that drives a signal electrode (segment electrode) of the liquid crystal display panel 21. The scan driving circuit 22a and the signal driving circuit 22b respectively drive the electrodes of the liquid crystal display panel 21 according to a control signal from the drive control circuit 24, so that images are displayed on the liquid crystal display panel 21.
The power supply control circuit 23 includes a boosting circuit 30 and a power supply selection circuit 31. A power supply voltage V from the secondary battery 10 is supplied to both of the boosting circuit 30 and the power supply selection circuit 31. The power supply selection circuit 31 is supplied not only with the power supply voltage V from the secondary battery 10 but also with a boosted voltage Vs that is boosted by the boosting circuit 30. The power supply selection circuit 31 selects one of the power supply voltage V supplied from the secondary battery 10 and the boosted voltage Vs supplied from the boosting circuit 30 and supplies the selected voltage to the liquid crystal driving circuit 22 as a driving voltage Vz for driving the liquid crystal display panel 21. The selection by the power supply selection circuit 31 is controlled by the control device 11, and the selection is switched when a control signal C2 is supplied from the control device 11. Further, the boosting circuit 30 is also controlled by the control device 11, and the boosting circuit 30 starts operating when a control signal C1 is supplied from the control device 11.
The boosting circuit 30 is described hereinafter in detail with reference to FIG. 7. FIG. 7 is a circuit diagram showing the boosting circuit 30. As shown in FIG. 7, the boosting circuit 30 is a charge pump boosting circuit that includes a double booster 55 including switches SW1 and SW2 and a pump capacitor Cp and a smoothing capacitor 56 including a backup capacitor Cb.
The switches SW1 and SW2 are interlock switches, and when the control signal C1 is supplied from the control device 11, the state where the switches SW1 and SW2 are connected to the side a and the state where the switches SW1 and SW2 are connected to the side b are switched according to a prescribed operating frequency. As a result that the switches SW1 and SW2 are repeatedly switched, the state where the voltage V is charged to the capacitor Cp and the state where the capacitor Cp and the capacitor Cb are connected in parallel and charge is transferred from the capacitor Cp to the capacitor Cb are switched. As a result that those states are repeatedly switched, the power supply voltage that is supplied from the secondary battery 10 is boosted double.
In the power supply circuit according to prior art, when the power supply voltage V of the secondary battery 10 falls below the driving voltage Vz for driving the liquid crystal display panel 21, the output V of the secondary battery 10 and the output Vs of the boosting circuit 30 are switched by the power supply selection circuit 31. It is thereby possible to use the secondary battery 10 for a longer time.
As described above, in the power supply circuit according to prior art, the output of the secondary battery 10 and the output of the boosting circuit 30 are switched by the power supply selection circuit 31. However, the present inventor has found that the following problem occurs when switching between a plurality of boosting circuits with different output voltages by the power supply selection circuit in the power supply circuit according to prior art.
In the power supply circuit according to prior art, a smoothing capacitor is connected to the output end of a boosting circuit. Particularly, a boosting circuit that is used in a display driver for mobile apparatus requires a high-capacitance external capacitor. When switching the output of the boosting circuit by the power supply selection circuit, the charge charged in the smoothing capacitor is abandoned, which is significant waste of power.
Further, when switching from a boosting circuit that generates a low boosted voltage to a boosting circuit that generates a high boosted voltage, the power supply voltage rises abruptly at the output of the power supply selection circuit, which may cause an adverse effect on loads. An abrupt change in the power supply voltage of the entire circuit may cause malfunction such as an abrupt flow of current into a part of the circuit.